Nickel based superalloy

ABSTRACT

A nickel based single crystal superalloy comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-8 wt % tungsten, 3.5-6 wt % rhenium, 5-6.5 wt % aluminium, 6.5-8.5 wt % tantalum, 0-0.2 wt % hafnium and the balance nickel plus incidental impurities. The nickel based single crystal superalloy is suitable for use as a gas turbine engine turbine blade or turbine vane. It is of particular us on cooled turbine blades and turbine vanes which have ceramic thermal barrier coatings, because the superalloy is compatible with the ceramic thermal barrier coating to minimise spalling. The superalloy has better mechanical properties than other second-generation single crystal superalloys and has better creep strength and oxidation resistance.

The present invention relates to nickel based superalloys, particularly to nickel based single crystal superalloys, or particularly nicked based single crystal superalloys for use as turbine blades, turbine vanes, turbine seals and combustor components of gas turbine engines, but they may be used in internal combustion engines etc.

Nickel based single crystal superalloys have been developed to provide improved high temperature mechanical properties such as creep strength. However, there are many other important properties, which need to be optimised to a high level in order for a nickel based single crystal superalloy to be acceptable for use in a gas turbine engine. Other properties, which need to be optimised, are density, resistance to oxidation, resistance to corrosion, compatibility with protective coatings, heat treatment response and castability.

There are three generations of nickel based single crystal superalloys which differ by the amount of the key element rhenium. The first generation of nickel based single crystal superalloys contained no rhenium, examples of these are disclosed in published UK patent application nos. GB2039296A, GB2073774A, GB2105369A, GB2106138A and GB2151659A. The first generation of nickel based single crystal superalloys have densities of 7.9 to 8.7 gm per cm³. The second generation of nickel based single crystal superalloys contained about 3 wt % rhenium, examples of these are disclosed in published European patent application nos. EP0155827A and EP0208645A. The second generation of nickel based single crystal superalloys have densities of 8.7 to 8.9 gm per cm³. The second generation of nickel based single crystal superalloys have a benefit in creep strength capability of about 30° C. over the first generation of nickel based single crystal superalloys. The third generation of nickel based single crystal superalloys contained about 6 wt % rhenium, examples of these are disclosed in U.S. Pat. Nos. 5,366,695 and 5,270,123 and published European patent application no. EP0848071A. The third generation of nickel based single crystal superalloys have densities of 8.9 to 9.1 gm per cm³. The third generation of nickel based single crystal superalloys have a benefit in creep strength capability of about 30° C. over the second generation of nickel based single crystal superalloys. However, the third generation of nickel based single crystal superalloys may also have narrow heat treatment ranges, which incur manufacturing costs. The high levels of strengthening elements, such as rhenium and tungsten, that are added also have a detrimental effect on the metallurgical stability of the third generation of nickel based single crystal superalloys.

Thus it is seen that the increase in creep strength is to the detriment of the density and the cost of the superalloy. An increase in density of the turbine blades and turbine vanes makes the gas turbine engine heavier and also results in a requirement to make the turbine rotor disc stronger to carry the heavier turbine blades, which also results in an increase in the weight of the turbine rotor disc.

The metal temperature of the investment cast turbine blades, or turbine vanes, is frequently reduced by air cooling of the turbine blades, or turbine vanes. It is often the case that cooled turbine blades, or turbine vanes, are protected by ceramic thermal barrier coatings. A major concern with a ceramic thermal barrier coating is that the ceramic thermal barrier coating will spall prematurely during engine service. The adherence of a ceramic thermal barrier coating is influenced by many factors, but a major factor is the composition of the superalloy substrate on which the ceramic thermal barrier coating is deposited.

The present invention seeks to provide a novel nickel based single crystal superalloy, which has a capability greater than the second generation of nickel based single crystal superalloys, but which is metallurgically stable without excessive costs.

Accordingly the present invention provides a nickel based single crystal superalloy comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-8 wt % tungsten, 3.5-6 wt % rhenium, 5-6.5 wt % aluminium, 6.5-8.5 wt % tantalum, 0-0.2 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the nickel based single crystal superalloy comprises 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-7.5 wt % tungsten, 3.5-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the nickel based single crystal superalloy comprises 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the nickel based single crystal superalloy comprises 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the nickel based single crystal superalloy comprises 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 2-2.4 wt % molybdenum, 4.3-4.9 wt % tungsten, 3.8-4.4 wt % rhenium, 5.5-6.1 wt % aluminium, 7.2-7.8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The nickel based single crystal superalloy may comprise 8 wt % cobalt, 4 wt % chromium, 2.2 wt % molybdenum, 4.6 wt % tungsten, 4.1 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium and the balance nickel plus incidental impurities.

Alternatively the nickel based single crystal superalloy comprises 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The nickel based single crystal superalloy may comprise 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The nickel based single crystal superalloy may comprise 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 6.5-7.5 wt % tungsten, 5.0-5.6 wt % rhenium, 5.5-6.1 wt % aluminium, 7.0-8.0 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The nickel based single crystal superalloy may comprise 8 wt % cobalt, 4 wt % chromium, 7 wt % tungsten, 5.3 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium and the balance nickel plus incidental impurities.

The present invention also provides a cast single crystal nickel based superalloy article comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-8 wt % tungsten, 3.5-6 wt % rhenium, 5-6.5 wt % aluminium, 6.5-8.5 wt % tantalum, 0-0.2 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the cast single crystal nickel based superalloy article comprises 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 0.2.4 wt % molybdenum, 4-7.5 wt % tungsten, 3.5-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the cast single crystal nickel based superalloy article comprises 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the cast single crystal nickel based superalloy article comprises 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the cast single crystal nickel based superalloy article comprises 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 2-2.4 wt % molybdenum, 4.3-4.9 wt % tungsten, 3.8-4.4 wt % rhenium, 5.5-6.1 wt % aluminium, 7.2-7.8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The cast single crystal nickel based superalloy article may comprise 8 wt % cobalt, 4 wt % chromium, 2.2 wt % molybdenum, 4.6 wt % tungsten, 4.1 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium and the balance nickel plus incidental impurities.

Alternatively the cast single crystal nickel based superalloy article comprises 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The cast single crystal nickel based superalloy article may comprise 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The cast single crystal nickel based superalloy article may comprise 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 6.5-7.5 wt % tungsten, 5.0-5.6 wt % rhenium, 5.5-6.1 wt % aluminium, 7.0-8.0 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.

The cast single crystal nickel based superalloy article may comprise 8 wt % cobalt, 4 wt % chromium, 7 wt % tungsten, 5.3 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium and the balance nickel plus incidental impurities.

Preferably the cast single crystal article comprises a turbine blade, a turbine vane or a combustor component.

Preferably the cast single crystal article comprises at least one internal passage for the flow of cooling fluid.

Preferably the cast single crystal article comprises a bond coating on the article and a ceramic thermal barrier coating on the bond coating.

Preferably the bond coating comprises a layer of alumina.

Preferably the bond coating comprises a layer comprising platinum enriched gamma prime phase and platinum enriched gamma phase.

The present invention will be more fully described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a bar chart comparing high temperature oxidation resistance at 1100° C. for two nickel based single crystal superalloys according to the present invention and a prior art nickel based single crystal superalloy.

FIG. 2 is a bar chart showing the average thermal barrier coating spallation time against temperature for two nickel based single crystal superalloys according to the present invention and a prior art nickel based single crystal superalloy.

FIG. 3 is a graph showing the ratio of time to 1% creep strain for two nickel based single crystal superalloys according to the present invention and a prior art nickel based single crystal superalloy against temperature.

FIG. 4 is a graph showing the ratio of creep rupture lives for two nickel based single crystal superalloys according to the present invention and a prior art nickel based single crystal superalloy against temperature.

A nickel based single crystal superalloy with high temperature mechanical properties, high temperature oxidation resistance and metallurgical stability better than second-generation nickel based single crystal superalloys but which has compatibility with ceramic thermal barrier coatings.

The compositions of the nickel based single crystal superalloys of the present invention were defined to give an increase in mechanical strength over the second generation nickel based single crystal superalloy CMSX4, in particular by increasing the level of rhenium relative to CMSX4, e.g. to above 3 wt %. The chromium level was decreased relative to CMSX4 to ensure metallurgical stability against formation of the undesirable topologically close packed (TCP) phases whilst retaining sufficient resistance against low temperature oxidation. The titanium level was set to zero because this element degrades the oxidation resistance of cast nickel based single crystal superalloys, in particular it promotes spalling of ceramic thermal barrier coatings. The rhenium level was set at a maximum of 5.3 wt % to avoid excessive raw material costs. The density of the nickel based single crystal superalloy was maintained below 9 gm per cm³ to avoid excessive stresses on the turbine discs and increases in weight in the gas turbine engine.

Materials modelling capability was used to assess melting temperature and the gamma prime solvus temperature to ensure a sufficient heat treatment window for cost effective practical production of the nickel based single crystal superalloy components, e.g. turbine blades, turbine vanes etc. Materials modelling capability was used to assess the propensity for formation of the damaging topologically close packed (TCP) phases to ensure that the nickel based single crystal superalloys has sufficient metallurgical stability for application as turbine blades, turbine vanes etc in gas turbine engines.

A number of alloys were prepared as shown in Table 1, and Table 1 also includes known superalloy CMSX4 and CMSX10 of Cannon-Muskegon Corporation, of 2875 Lincoln Street, Muskegon, Mich., USA and described in European patent application EP0155827A and U.S. Pat. No. 5,366,695 respectively. TABLE 1 Element (wt %) Alloy Co Cr Mo W Re Al Ti Ta Nb Hf Ni A 8.0 4.0 2.2 4.6 4.1 5.8 0 7.5 0 0.1 bal B 8.0 4.0 2.2 4.6 5.3 5.8 0 7.5 0 0.1 bal C 8.0 4.0 2.2 5.8 4.1 5.8 0 7.5 0 0.1 bal D 8.0 4.0 2.2 5.2 4.7 5.8 0 7.5 0 0.1 bal E 8.0 4.0 2.2 5.2 4.7 5.8 0 6.5 0.6 0.1 bal F 8.0 4.0 2.2 5.2 4.7 5.8 0 5.2 1.2 0.1 bal G 8.0 4.0 3.5 4.0 4.0 5.8 0 6.5 0.6 0.1 bal H 8.0 4.0 0.0 7.0 5.3 5.8 0 7.5 0 0.1 bal I 8.0 5.5 2.0 4.0 5.3 5.8 0 7.5 0 0.1 bal J 2.0 4.0 2.2 5.2 4.7 5.8 0 7.5 0 0.1 bal K 12.0 4.0 2.2 5.2 4.7 5.8 0 7.5 0 0.1 bal L 8.0 2.5 2.2 4.6 5.3 6.0 0 7.5 0 0.1 bal CMSX4 9.5 6.5 0.6 6.4 3.0 5.6 1.0 6.5 0 0.1 bal CMSX10 2.7 2.0 0.4 5.3 6.3 5.65 0.2 5.8 7.9 0.04 bal

The nickel based single crystal superalloys in Table 1, except for CMSX10, were subjected to a series of tests. The tests comprised testing to establish a heat treatment cycle, creep testing, oxidation testing, thermal barrier coating spallation testing, thermal exposure to study formation of topologically close packed (TCP) phases and detailed metallurgical examination.

The tests showed that superalloys A and H in particular have advantages over the prior art alloy CMSX4. Both superalloys A and H can be heat treated and homogenised at high temperatures to dissolve the gamma prime eutectic phase and this helps optimise the properties of the superalloys.

The superalloys in Table 1 were initially tested for oxidation resistance. Cyclic oxidation testing was performed on a burner rig, the cycling rate was 4 cycles per hour and 0.25 ppm of simulated sea salt was added to the gas flow to simulate operation in marine environment. The measure of amount of attack on the superalloy samples is by metal loss per surface and the data is shown in FIG. 1 for testing at a temperature of 1100° C. for superalloys A, H and CMSX4. After 400 cycles of testing the superalloy samples were sectioned and the metal loss measured using a travelling microscope. It can be seen that superalloys A and H have better oxidation resistance to CMSX4.

The superalloys in Table 1 were tested for compatibility with a known thermal barrier coating system by depositing about 8 μm of platinum onto the samples of the superalloy substrate and heat treating at 1150° C. to form a layer comprising platinum enriched gamma phase and platinum enriched gamma prime phase. This layer together with a layer of alumina which forms on the layer becomes a bond coating for a ceramic thermal barrier coating deposited by electron beam physical vapour deposition.

Samples of each superalloy were exposed in a still air furnace at temperatures of 1170° C., 1190° C., 1210° C., 1230° C. and 1250° C. until spallation of the ceramic thermal barrier coating occurred over more than 50% of the surface of the sample. The compatibility of the superalloys with the thermal barrier coating is shown in FIG. 2, this shows that superalloy H has significantly better compatibility with thermal barrier coatings than both superalloy A and CMXS4 across the temperature range tested. This also shows that superalloy A has a compatibility with thermal barrier coatings that is intermediate that of superalloy H and CMSX4 and superalloy A has a compatibility better than CMSX4 at some temperatures.

The creep performance can be expressed as the time to 1% creep strain under various conditions of stress and temperature. The ratio of the creep lives for 1% strain to the lives of CMSX4 across the temperature range are shown in FIG. 3 and this shows that both superalloys A and H are superior to CMSX4 across the temperature range and that superalloy H is superior to superalloy A, especially at 950° C.

The ratio of the creep rupture lives of superalloys A and H as a function of the creep rupture life of CMSX4 are shown in FIG. 4 and this shows that both of superalloys A and H are superior to CMSX4 across the temperature range and superalloy H is superior to superalloy A.

The metallurgical stability of the superalloys was assessed by detailed metallurgical examination of samples of the superalloy exposed in a furnace together with examination of the samples of the superalloys exposed to the creep tests. The examinations showed that both superalloys A and H were extremely stable. In particular superalloy A showed no evidence of topologically close packed (TCP) phase formation. Superalloy H showed very limited topologically close packed (TCP) phase formation after 240 hours exposure at 1100° C.

The present invention has shown that superalloys A and H are metallurgically stable, have superior properties to CMSX4 and are suitable for use as turbine blades and turbine vanes in aero gas turbine engines, marine gas turbine engines or for industrial gas turbine engines. Superalloy H has the advantage of better overall properties compared to superalloy A. However, superalloy A has the advantage of slightly improved metallurgical stability, lower rhenium content and hence lower cost and slightly lower density. Superalloy A has a density of 8.85 gm per cm³ and superalloy H has a density of 9.0 gm per cm³. The superalloys of the present invention also allow operation at a temperature of 15° C. to 25° C. higher relative to CMSX4.

Other suitable bond coatings may be used on the nickel based single crystal superalloy article, for example MCrAlY, aluminide, platinum aluminide, etc. The ceramic thermal barrier coatings may be deposited by other suitable methods for example sputtering, vacuum plasma spraying, air plasma spraying, chemical vapour deposition, etc. The ceramic thermal barrier coatings may comprise yttria-stabilised zirconia, ceria stabilised zirconia or other suitable ceramics. 

1. A nickel based single crystal superalloy comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-8 wt % tungsten, 3.5-6 wt % rhenium, 5-6.5 wt % aluminium, 6.5-8.5 wt % tantalum, 0-0.2 wt % hafnium and the balance nickel plus incidental impurities.
 2. A nickel based single crystal superalloy as claimed in claim 1 comprising 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-7.5 wt % tungsten, 3.5-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 3. A nickel based single crystal superalloy as claimed in claim 1 comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 4. A nickel based single crystal superalloy as claimed in claim 3 comprising 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 5. A nickel based single crystal superalloy as claimed in claim 4 comprising 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 2-2.4 wt % molybdenum, 4.3-4.9 wt % tungsten, 3.8-4.4 wt % rhenium, 5.5-6.1 wt % aluminium, 7.2-7.8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 6. A nickel based single crystal superalloy as claimed in claim 5 comprising 8 wt % cobalt, 4 wt % chromium, 2.2 wt % molybdenum, 4.6 wt % tungsten, 4.1 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium and the balance nickel plus incidental impurities.
 7. A nickel based single crystal superalloy as claimed in claim 1 comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 8. A nickel based single crystal superalloy as claimed in claim 7 comprising 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 9. A nickel based single crystal superalloy as claimed in claim 8 comprising 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 6.5-7.5 wt % tungsten, 5.0-5.6 wt % rhenium, 5.5-6.1 wt % aluminium, 7.0-8.0 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 10. A nickel based single crystal superalloy as claimed in claim 9 comprising 8 wt % cobalt, 4 wt % chromium, 7 wt % tungsten, 5.3 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium and the balance nickel plus incidental impurities.
 11. A cast single crystal nickel based superalloy article comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-8 wt % tungsten, 3.5-6 wt % rhenium, 5-6.5 wt % aluminium, 6.5-8.5 wt % tantalum, 0-0.2 wt % hafnium and the balance nickel plus incidental impurities.
 12. A cast single crystal nickel based superalloy article as claim in claim 11 comprising 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 0-2.4 wt % molybdenum, 4-7.5 wt % tungsten, 3.5-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 13. A cast single crystal nickel based superalloy article as claimed in claim 11 comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 14. A cast single crystal nickel based superalloy article as claimed in claim 13 comprising 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 2-2.4 wt % molybdenum, 4-5 wt % tungsten, 3.5-4.5 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 15. A cast single crystal nickel based superalloy article as claimed in claim 14 comprising 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 2-2.4 wt % molybdenum, 4.3-4.9 wt % tungsten, 3.8-4.4 wt % rhenium, 5.5-6.1 wt % aluminium, 7.2-7.8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 16. A cast single crystal nickel based superalloy article as claimed in claim 15 comprising 8 wt % cobalt, 4 wt % chromium, 2.2 wt % molybdenum, 4.6 wt % tungsten, 4.1 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium and the balance nickel plus incidental impurities.
 17. A cast single crystal nickel based superalloy article as claimed in claim 11 comprising 7-9 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5-6.5 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 18. A cast single crystal nickel based superalloy article as claimed in claim 17 comprising 7.5-8.5 wt % cobalt, 3.5-4.5 wt % chromium, 6-8 wt % tungsten, 4.8-5.8 wt % rhenium, 5.3-6.3 wt % aluminium, 7-8 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 19. A cast single crystal nickel based superalloy article as claimed in claim 18 comprising 7.7-8.3 wt % cobalt, 3.7-4.3 wt % chromium, 6.5-7.5 wt % tungsten, 5.0-5.6 wt % rhenium, 5.5-6.1 wt % aluminium, 7.0-8.0 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 20. A cast single crystal nickel based superalloy article as claimed in claim 19 comprising 8 wt % cobalt, 4 wt % chromium, 7 wt % tungsten, 5.3 wt % rhenium, 5.8 wt % aluminium, 7.5 wt % tantalum, 0.1 wt % hafnium, and the balance nickel plus incidental impurities.
 21. A cast single crystal nickel based superalloy article as claimed in claim 11 wherein the article comprises a turbine blade, a turbine vane or a combustor component.
 22. A cast single crystal nickel based superalloy article as claimed in claim 21 wherein the article comprises at least one internal passage for the flow of cooling fluid.
 23. A cast single crystal nickel based superalloy article as claimed in claim 21 wherein the article comprises a bond coating on the article and a ceramic thermal barrier coating on the bond coating.
 24. A cast single crystal nickel based superalloy article as claimed in claim 23 wherein the bond coating comprises a layer of alumina.
 25. A cast single crystal nickel based superalloy article as claimed in claim 23 wherein the bond coating comprises a layer comprising platinum enriched gamma prime phase and platinum enriched gamma phase.
 26. A nickel based single crystal superalloy comprising 7.8-8.2 wt % cobalt, 3.8-4.2 wt % chromium, 6.8-7.2 wt % tungsten, 5.1-5.5 wt % rhenium, 5.6-6.0 wt % aluminium, 7.7-7.9 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities.
 27. A cast single crystal nickel based superalloy article comprising 7.8-8.2 wt % cobalt, 3.8-4.2 wt % chromium, 6.8-7.2 wt % tungsten, 5.1-5.5 wt % rhenium, 5.6-6.0 wt % aluminium, 7.7-7.9 wt % tantalum, 0.05-0.15 wt % hafnium and the balance nickel plus incidental impurities. 